UPDATE 1-Washington is limited in its response to N. Korea over Sony hack
December 19, 2014 at 18:40 PM EST
WASHINGTON/SEOUL, Dec 19 (Reuters) - President Barack Obama's threat of a "proportional" response to North Korea's hacking of Sony Pictures shows once more the limits of Washington's options as it seeks to rein in one of the world's most unpredictable and isolated countries.